1. Field of the Invention
This invention relates to conjugates having reduced adverse systemic effects and methods for making and using the same.
2. Description of Related Art
Chemotherapy involves administering to a patient an active agent designed to modulate one or more cellular functions of a target cell associated with a disease or disorder. The modulating action can range from reducing an activity, such as down-regulating the expression of a gene, to suppressing the activity entirely. Frequently, but not necessarily, the target cell is a cancer cell, in which case the cellular functions modulated are those vital to cell survival, with the objective of affecting them to such an extent that the cell cannot survive and a cytocidal effect is achieved—that is, the active agent is a cytotoxin.
A common drawback of chemotherapy is the effect of the active agent on non-target cells as well as target cells, resulting in systemic (non-selective) adverse effects. In principle this drawback can be overcome by designing an active agent that affects only the target cell, but in practice absolute selectivity is rarely achieved.
An alternative approach is to covalently link the active agent via a linker moiety to a targeting moiety that has affinity for the target cell, forming a targeting moiety/linker moiety/active agent conjugate. The active agent is latently active: in its conjugated form it is inactive, but when released from the conjugate by cleavage of the linker moiety, it is active. The targeting moiety directs the conjugate to the target cell, after which the conjugate is internalized by endocytosis. As an illustration, the targeting moiety can be an antibody (particularly a monoclonal antibody or “mAb”) having specific affinity for a tumor-associated antigen (“TAA”) characteristic of a target cancer cell and the active agent is an anti-cancer drug. In the vernacular of the art, the active agent is referred to as a “warhead,” analogizing the conjugate to a military guided missile.
The linker moiety is designed to be stable outside of the target cell but unstable inside it (or, at least, more stable outside than inside). Cleavage of the linker moiety in response to conditions prevalent inside the target cell releases the active agent. Intracellular conditions triggering cleavage can be varied. The end destination of an endocytosed molecule is normally a lysosome inside the cell. The lysosomal environment is more acidic (typically about pH 5) than blood plasma (typically about pH 7.3), so that a linker moiety that is pH sensitive can be selectively cleaved inside a target cell. Also, a lysosome contains acid hydrolases, which are peptidases active at acidic pH's. A peptidic linker moiety that is a specific substrate for the acid hydrolases will be cleaved preferentially inside a lysosome. Or, a redox-potential sensitive linker moiety may be preferentially cleaved in response to a difference in redox potential.
Nevertheless, a certain amount of premature cleavage of the linker moiety is virtually unavoidable. Applying the rule of thumb of a 10× differential per pH unit in the rate of a first-order acid catalyzed reaction, a pH sensitive linker moiety will cleave in blood serum at about 1% of the lysosomal rate. Or, a peptidic linker moiety designed to be a specific substrate for a lysosomal acid hydrolase may be a non-specific substrate for a serum protease. Whenever premature cleavage occurs, the result is “leakage” of the active agent into the blood plasma and a consequent risk of a negative systemic effect. Such risk is especially serious where the active agent is toxic towards cells generally.
Thus, it is desirable to develop solutions to the problem of premature active agent release in a conjugate resulting in adverse systemic effects.